Uncaught Exception: The service "" has a dependency on a non-existent service ""

Hallo, habe analog zu anderen Plugins in der service.xml in einen Subscriber eine Klasse eingebunden, bekomme aber die Errormeldung:

Uncaught Exception: The service "WtCustomerRegistration\Subscriber\CustomerRegistration" has a dependency on a non-existent service "WtCustomerRegistration\Service\Mailer".

Was läuft da schief?
Hier ein Auszug aus der composer.json:

    "extra": {
        "shopware-plugin-class": "WtCustomerRegistration\\WtCustomerRegistration",
        "label": {
            "de-DE": "Product Registration Plugin",
            "en-GB": "Product Registration Plugin"
        }
    },

Hier die services.xml

<?xml version="1.0" ?>

<container xmlns="http://symfony.com/schema/dic/services"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>

        <service id="WtCustomerRegistration\Subscriber\CustomerRegistration">
            <argument type="service" id="Shopware\Core\System\SystemConfig\SystemConfigService"/>
            <argument type="service" id="WtCustomerRegistration\Service\Mailer"/>
            <tag name="kernel.event_subscriber"/>
        </service>

    </services>
</container>

Hier die Mailler Klasse:

<?php

namespace WtCustomerRegistration\Service;

/**
 *
 */
class Mailer
{

}

Und hier der Subscriber:

<?php declare(strict_types=1);

namespace WtCustomerRegistration\Subscriber;

use JetBrains\PhpStorm\NoReturn;
use Shopware\Storefront\Event\StorefrontRenderEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Shopware\Core\System\SystemConfig\SystemConfigService;
use WtCustomerRegistration\Service\Mailer;

/**
 *
 */
class CustomerRegistration implements EventSubscriberInterface
{
    private SystemConfigService $systemConfigService;
    private Mailer $mailer;

    public function __construct(SystemConfigService $systemConfigService,Mailer $mailer)
    {
        $this->systemConfigService = $systemConfigService;
        $this->mailer=$mailer;
    }

    public static function getSubscribedEvents(): array
    {
        // Return the events to listen to as array like this:  <event to listen to> => <method to execute>
        return [
            StorefrontRenderEvent::class => 'onStoreFrontLoaded',
        ];
    }

    #[NoReturn] public function onStoreFrontLoaded(StorefrontRenderEvent $event): void
    {
        // Do something
        // E.g. work with the loaded entities: $event->getEntities()
    }
}

Was läuft da schief?

Ich bin selber drauf gekommen: Ich muss die Mailer Klasse in der services.xml gesondert registrieren. So sieht das dann aus:

<?xml version="1.0" ?>

<container xmlns="http://symfony.com/schema/dic/services"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>

        <service id="WtCustomerRegistration\Subscriber\CustomerRegistration">
            <argument type="service" id="Shopware\Core\System\SystemConfig\SystemConfigService"/>
            <argument type="service" id="WtCustomerRegistration\Service\Mailer"/>
            <tag name="kernel.event_subscriber"/>
        </service>
        <service id="WtCustomerRegistration\Service\Mailer">
        </service>
    </services>
</container>

Wenn in der Mailer Klasse zusätzliche Klassen oder Interfaces verwendet werden, müssen die in dem soeben zugefügten Mailer Dienst integriert werden